The preparation of printed circuits, including multilayer printed circuits is known. Powdered materials such as particulate metals have been applied to surfaces having imagewise tacky and nontacky areas by a number of toning methods to produce printed circuit patterns. Representative methods are disclosed in U.S. Pat. Nos. 3,060,024; 3,649,268; 4,054,479, 4,054,483, 4,157,407 and 4,234,626. After the particulate metal is applied to the tacky image areas and unwanted particles are removed, e.g., mechanically, from the nontacky image areas, the circuit is formed by one of several additive techniques including fusion of metal particles, electroless plating, electroplating, etc. The printed circuits formed by these additive processes are useful, but the processes have certain disadvantages. While the printed circuits may possess some flexibility, it is necessary in the preparation of some products, e.g., electronic switches, etc. to provide highly flexible printed circuits which are capable of being bent continuously over a small radius. Electronic switches can be prepared by screen printing the circuitry onto flexible film supports by means of electrically conductive ink. Such printing must be done uniformly in the correct position for each circuit and then dried properly. The process requires skillful printers and special printing equipment and dryers. The types of inks used for preparing electronic switches do not always dry uniformly to give reproducible conductivity and the adhesion of the ink to its substrate is sometimes insufficient.
An object of this invention is to provide a process, including a continuous process, whereby flexible printed circuits can be prepared by additive procedures such as electroless plating and soldering, using flexible photohardenable elements which are capable of being bent over a small radius; the photohardenable composition being capable of surviving the plating and soldering and providing reliable conductivity and adhesion. Another object of this invention is to provide a process for the preparation of electronic or membrane switches using flexible photohardenable elements and additive techniques for the formation of the circuits.